Test apparatus for the testing of circuit boards may be divided basically into two groups: the group of finger testers (flying probers) and the group of parallel testers. Parallel testers are test devices which, by means of an adapter, are able to contact simultaneously all or at least most contact points of a circuit board to be tested. Finger testers are test devices for the testing of non-componented or componented circuit boards which scan the individual contact points in sequence using two or more test fingers.
The test fingers are usually mounted on a slide, along which cross-bars are able to traverse, while the cross-bars in turn are guided and able to traverse on guide rails. Each test finger has a swivel arm, on the end of which a contact tip for contacting the circuit board is formed. By traversing the slide and rotating the swivel arm, the test fingers may therefore be positioned with their contact tips at any desired point in a test field, generally rectangular. To contact a contact point of a circuit board to be tested, either the slide is designed to be able to move vertically on the cross-bar or the test probe is designed to move vertically on the slide, so that the test finger may be placed from above or below on the contact point of the circuit board, the circuit board test point.
A finger tester is described in EP 0 468 153 A1 and a method for testing circuit boards by means of a finger tester is described in EP 0 853 242 A1. Test probes for finger testers are disclosed in EP 1 451 594 B1; US 6,384,614 B1; WO 03/096037 A1; and EP 0 990 912 A2.
A finger tester of this kind is usually provided with at least two traversable contact fingers, so as to be able to reach test points spaced apart along the cross-bar axis, and several cross-bars are provided above the test field and (at any rate for the testing of circuit boards printed on both sides) several cross-bars below the test field, for reaching test points spaced apart in a second coordinate direction at right-angles to the cross-bar axis. (In principle, a circuit board may be tested with only a single test finger, if capacitive measurement is used. In this case, in theory, only one test finger is needed. It is however preferred to have two test fingers mounted on a rail, since an ohmic measurement requires a closed circuit, and therefore two test fingers are needed). For the use of automated test methods involving a finger tester it is important to know precisely the relative position of the cross-bars both to one another and to the test field. In the finger tester described in EP 0 466 153, the cross-bars are mounted in a stand, at right-angles to the direction of traversing of the test finger, so as to be traversable. Because of play in the traversing of the cross-bars, often unavoidable, the spacing of the cross-bars has a certain fluctuation width and, depending on the drive system used, slipping may cause the spacing of the cross-bars to move out of a preset field of tolerance, requiring readjustment. The associated adjustment and calibration processes for at least four cross-bars are time-consuming and often lead to inaccuracies.
Also known are finger testers in which the cross-bars are fixed immovably in a stand. Since the cross-bars are suspended in a stand both above and below the circuit board, they must be carefully and individually adjusted. The length of the swivel arms may be optimized in respect of a distance between the cross-bars, so that the test field between the cross-bars may be reached in the optimal manner. If such a finger tester has to be adapted to changed process conditions, perhaps because the scanning density needs to be increased or reduced, then it is necessary to fit additional cross-bars, or to remove or refit existing cross-bars. This also involves considerable effort for adjustment of the cross-bars relative to one another and to the test field, and for calibration of the software.